Electrical and optical characterizations of β-Ga 2 O 3 ...

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Du, C. Luan, Z. Li, C. Zhao, H. xiao and J. ma, RSC Adv., 2014, DOI: 10.1039/C4RA02479F.

http://dx.doi.org/10.1039/C4RA02479F

http://pubs.rsc.org/en/journals/journal/RA

1

Electrical and optical characterizations of β-Ga2O3:Sn films deposited on

MgO (110) substrate by MOCVD

Wei Mi, Xuejian Du, Caina Luan, Hongdi Xiao, Jin Ma*

School of Physics, Shandong University, Jinan 250100, PR China

*Corresponding author. Tel.: +86 531 88361057; fax: +86 531 88564886.

E-mail address: [email protected] (J. Ma)

Abstract

Tin-doped β-Ga2O3 (β-Ga2O3:Sn) films doped with different tin concentrations

were deposited on MgO (110) substrates by metal organic chemical vapor deposition

(MOCVD) at 700 °C. The effect of doping on the structural, electrical and optical

properties of the films was investigated. The 10% Sn-doped film exhibited the best

electrical conductive properties with the lowest resistivity about 5.21×10-2 Ω·cm,

which is over ten orders of magnitude lower than the un-doped film. Micro-structural

analysis revealed that the film with 10% Sn content had a clear in-plane relationship

of β-Ga2O3 (100) || MgO (110) with β-Ga2O3 ( 201 ) || MgO (111). The average

transmittance of the samples in the visible range exceeded 87% and the optical band

gap of the films varied from 4.12 to 4.80 eV.

Keywords: β-Ga2O3:Sn films; MOCVD; Electrical characterization; Band gap.

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View Article OnlineDOI: 10.1039/C4RA02479F


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I. Introduction

β-type gallium oxide belongs to the group of the transparent conductive oxides

(TCOs). Comparing with the traditional TCOs such as ZnO1, SnO22 and In2O3

3,

β-Ga2O3 has a wider band gap (Eg~4.8 eV4,5), which can be used in short wavelength

light emitting devices6, deep ultraviolet (UV) photodetectors7 and gas sensors8. The

crystal structure of β-Ga2O3 is monoclinic with space group C2/m. The lattice

parameters are a=12.23 Å, b=3.04 Å, c=5.80 Å, β=103.7° (PDF#43-1012)9. However,

pure β-Ga2O3 films show poor conductivity due to the large energy band gap10. It is

needed to increase the conductivity by doping method. There have been many reports

about the β-Ga2O3 materials doping with different elements. 10 mol % Sn-doped

β-Ga2O3 single crystals with the resistivity of 1.5×104 Ω·cm were fabricated by the

floating zone method11. The electrical conductivity of β-Ga2O3 single crystals can be

intentionally controlled over three orders of magnitude by Si doping12.

Europium-doped gallium oxide films were grown on sapphire substrates by pulsed

laser deposition and were characterized using optical techniques13. However, most of

the researches were focused on the β-Ga2O3 bulk materials14, polycrystalline films15

and nanowires16-18. Compared with the polycrystalline films and nanowires, single

crystalline epitaxial β-Ga2O3:Sn films have many advantages such as better

uniformity, stability and compactness. There were few reports about the electrical

characterization of Sn-doped β-Ga2O3 epitaxial films, especially by means of

MOCVD technique In this letter, β-Ga2O3:Sn films with different Sn concentrations

were fabricated on MgO (110) substrates at 700 °C by MOCVD. The results show

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that Sn is an effective n-type dopant for β-Ga2O3 films and the conductivity of the

film can be reduced by over ten orders of magnitude by Sn doping. MgO (110)

substrate is very suitable for growing β-Ga2O3 single crystal epitaxial films19, and its

price is much cheaper compared with gallium oxide wafer. MOCVD technique has

the advantages of growth epitaxial films and precise control of films composition,

which is suitable for both scientific research and industrial production. This work

provides a feasible method for improving the performance of β-Ga2O3 film materials,

thus it will be favorable for the application of Ga2O3 in fields of transparent electronic

devices and UV light emitting devices. The structural, electrical and optical properties

of the β-Ga2O3:Sn films have been investigated in detail.

II. Experimental details

The films were prepared on MgO (110) substrates (double-face polished, thickness:

0.5 mm) by a high vacuum MOCVD system with two separated gas flows.

Trimethylgallium Ga(CH3)3, and tetraethyltin Sn(C2H5)4, were used as the

organometallic (OM) source. High purity N2 (purity, 9N) passed through the OM

bubblers and delivered the OM vapor to the reactor. High purity O2 (purity, 5N) with

the flow rate of 50 sccm was used as oxidant and injected into the reactor using a

separate delivery line. The pressure of the reactor was kept at 20 Torr and the

substrate temperature was 700 oC. The flow rate of Ga(CH3)3 was 3.64×10-6 mol/min.

The flow rates of Sn(C2H5)4 were kept at 3.60×10-8, 1.08×10-7, 1.80×10-7, 2.88×10-7,

3.60×10-7, 3.96×10-7 and 4.32×10-7 mol/min, corresponding to 1%, 3%, 5%, 8%, 10%,

11% and 12% (atomic ratio) Sn concentrations, respectively.


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The crystalline structure and the epitaxial relationship of the deposited films were

measured by X-ray diffraction (XRD) and high resolution transmission electron

microscopy (HRTEM). A Rigaku D/max-rB X-ray diffractometer with Cu Ka1

radiation was used for the XRD measurements. The HRTEM and selected area

electron diffraction (SAED) measurements were performed on the cross-sectional

sample using a Tecnai F20 transmission electron microscope at 200 kV. The

cross-section and surface micrograph were obtained using a FEI Nova NanoSEM450

scanning electron microscope (SEM). The resistivity of the films was measured using

a ZC-36 high resistance meter. The Hall mobility and carrier concentration were

determined by the East changing ET9000 Hall measurement system. Optical

transmittance measurements were performed with a TV-1901 double-beam

UV–vis–NIR spectrophotometer in the wavelength range from 200 to 800 nm.

III. Results and discussion

Fig. 1 is the XRD patterns of β-Ga2O3:Sn films with various amounts of

Sn-doping from 1% to 12 mol%. In addition to the strong substrate peaks of MgO

(440) located at about 64°, a major diffraction peak of β-Ga2O3 (400) was observed,

indicating an out-of-plane orientation of β-Ga2O3 (100) || MgO (110). As the Sn

content increased, the β-Ga2O3 (400) peak becomes weaker. For the 12% Sn doped

sample, the β-Ga2O3 (400) diffraction peak cannot be seen clearly. The full width at

half maximum (FWHM) of the β-Ga2O3 (400) peak is 0.8, 1.0, 1.2, 1.6 and 2.4°

corresponding to the samples with Sn contents of 1%, 3%, 5%, 8% and 10%,

respectively, which reveals the degradation of the crystalline quality of the films. The


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radius of Sn4+ is bigger than that of Ga3+, when some of the Ga3+ are replaced by Sn4+,

the order of the β-Ga2O3 lattice is affected. So the crystalline quality of the films

decreases as the increasing of Sn contents. Further, when too many Sn element into

the film, gallium oxide lattice is destroyed and lead to the crystalline quality of film

serious decline.

Fig. 2 exhibits the resistivity of the β-Ga2O3:Sn films as a function of Sn

concentration. The undoped β-Ga2O3 film with the highest resistivity of about 109

Ω·cm is used as a comparison. With the increase of Sn content, the resistivity of the

sample decreases rapidly. The minimum resistivity value of about 5.21×10-2 Ω·cm is

obtained for the 10% Sn-doped sample. Then the resistivity increases as the Sn

content further raise to 11% and 12%. The decrease of resistivity can be explained by

the presence of the Sn4+ ion. When some of the Ga3+ ions in the lattice are replaced by

Sn4+ or some of the Sn ions as interstitial atoms are located in the lattice, conduction

electrons can be produced, thus the resistivity of films is reduced. However, as Sn

concentration increases further (over 10%), the crystalline quality of the films

becomes poor. Consequently, the defects inside the films lead to the enhancement of

the carrier scattering effect, and also the doping efficiency drops. As a result, the

electrical resistivity increased markedly. The variations of resistivity, carrier

concentration and mobility for the samples with 10% and 11% Sn doping levels are

summarized in Table 1. It can be seen that the carrier density and mobility both drop

obviously as the Sn content increases from 10% to 11%, resulting in the increase,

resulting in the increase of the resistivity.


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The temperature dependent Hall measurements are performed on 10% Sn-doped

film in vacuum (<10 Pa) and the results are shown in Fig. 3. The electrical properties

of the film exhibit a semiconductor-like behavior. As the temperature rises from low

to room temperature, the resistivity and Hall mobility decrease, and the carrier

concentration increases. At low temperature, the Hall mobility stays at a high level,

which can be attributed to the weak lattice scattering at the very low temperature. As

the temperature rises up to room temperature, the lattice vibration aggravates, leading

to the enhancement of phonon scattering and the decrease of Hall mobility. These

results demonstrate that Sn is an effective n-type dopant for β-Ga2O3 films and the

resistivity can be intentionally reduced by over ten orders of magnitude by Sn doping.

The cross-section area and surface morphologies of the 10% Sn-doped β-Ga2O3 film

are revealed by the SEM micrographs. From Fig. 4 (a), the obvious boundary between

the substrate and film can be observed, and the thickness of the film is about 470 nm.

In Fig. 4 (b), an orderly surface with well-defined crystallites is obtained. The

boundaries of each crystalline particle can be seen clearly.

Further structural properties of the sample with 10% Sn content are studied by

HRTEM. Fig. 5 shows the HRTEM and SAED micrographs of the interface between

the film and substrate. The incident electron beam is parallel to the [110 ] direction of

the MgO substrate. In the HRTEM image, the spacings of the observed lattice planes

in the film area are about 0.297 nm and 0.468 nm, corresponding to the β-Ga2O3 (400)

and ( 201 ) planes, respectively. For the substrate, the interplane spacings marked by

the white lines are 0.144 and 0.234 nm which are consistent with MgO (440) and


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(222), respectively. The diffraction spots of β-Ga2O3 (400), β-Ga2O3 ( 201 ), MgO

(440), MgO (222) can be seen clearly in the SAED micrograph. The results of

HRTEM and SAED show a clear orientation relationship of β-Ga2O3 (100) || MgO

(110) with β-Ga2O3 ( 201 ) || MgO (111).

Fig. 6 shows the XPS spectra for the β-Ga2O3:Sn film with 10% Sn content. Fig. 6

(a) is the survey spectrum of the film, which indicates the peaks of C1s, Ga2p, O1s,

Sn3d, Ga3s and Ga3d. Fig. 6 (b), (c) and (d) are the high resolution spectra of Ga2p,

Sn3d and O1s peaks, respectively. In Fig. 6 (b), two symmetrical peaks of Ga2p1/2

located at 1144.1 eV and Ga2p3/2 located at 1117.3 eV can be observed. The

separation distance between these two peaks is about 26.8 eV, which is in good

agreement with the binding energy of the Ga2p (∆ = 26.8 eV)20, 21. In Fig. 6 (c), the

tin core levels Sn3d5/2 and Sn3d3/2 observed at 486.4 and 494.8 eV, respectively, with

a peak-to-peak separation of 8.4 eV confirm the formation of Sn22. The O1s peak is

located at about 530.5 eV. Using the Gaussian Fitting method, the peak can be

resolved into two separate peaks. The signal at 530.5 eV is attributed to lattice oxygen

in the β-Ga2O3:Sn film23, 24. The other peak at 531.7 eV can be regard as the C/O or

OH– adsorbed species on the surface25. The compositional ratio of Sn-to-Ga is about

12%, which is larger than the expected value of 10%. The analysis above illustrates

that the obtained film is Sn-doped β-Ga2O3.

The optical transmittance spectra for the β-Ga2O3:Sn films as a function of

wavelength in the range of 200–800 nm are shown in Fig. 7 (a). The average

transmittance of all the films in visible rage exceeded 87%. For the direct band gap


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transition semiconductors, the absorption coefficient α and optical band gap (Eg) are

related by αhν=A(hν–Eg)1/2 26, 27. Where A is a constant, ν is the frequency of the

incident photon and h is the planck's constant. The optical band gap can be estimated

by extrapolating the straight line portion of this plot to the energy axis. The optical

gaps for the films as a function of the Sn content are shown in Fig. 7 (b). As the Sn

content increases, Eg decreases in initial with a minimum value of 4.12 eV obtained

for the 10% Sn-doped sample, and then the Eg increases. The variation trend of Eg is

consistent with that of resistivity. The decrease of Eg can be attributed to two reasons:

first the band gap of SnO2 is narrower than that of β-Ga2O3; second the band gap

narrowing effect28, 29. For the heavily doped semiconductors, the band tail and

impurity band will be generated; as the doping concentration increases from 0 to 10%,

the band tail and impurity band extended, which makes the band tail and impurity

band overlap. As a result to the actual width of the forbidden band becomes narrower.

But as the Sn concentration increases further (over 10%), the crystalline quality

declined obviously, resulting in the broadening of the band gap.

IV. Conclusions

In conclusion, β-Ga2O3:Sn films have been deposited on MgO (110) substrate by

MOCVD at 700 °C. Sn is an effective n-type impurity and can reduce the resistivity

by over ten orders of magnitude. The sample with 10% Sn content has the lowest

resistivity of about 5.21×10-2 Ω·cm with a carrier density of 3.71×1019 cm-3. The

micro-structure analysis of the 10% Sn-doped sample reveal a clear orientation

relationship of β-Ga2O3 (100) || MgO (110) with β-Ga2O3 ( 201 ) || MgO (111). The


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average transmittance of all the samples exceeded 87% in visible range. The film

grown with 10% Sn content has the minimum optical band gap of about 4.12 eV.

The variation trend of Eg is consistent with that of resistivity. The deposition of high

quality β-Ga2O3:Sn film with excellent electrical properties can be used in many fields

such as short wavelength light emitting devices, transparent transistors, quantum-well

devices and solar cells.

Acknowledgements

This work is financially supported by the National Natural Science Foundation of

China (Grant No. 51072102).

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Figure captions

Fig. 1 XRD spectra of the Ga2O3 :Sn films with different doping levels.

Fig. 2 The resistivity of the Ga2O3 films with different Sn concentrations.

Fig. 3 Resistivity (ρ), Carrier concentration (n) and Hall mobility (µ) of β-Ga2O3 films

with 10% Sn doped as a function of reciprocal temperature.

Fig. 4 The SEM image for the 10% Sn-doped β-Ga2O3 film: (a) the cross-section area

and (b) surface.

Fig. 5 Cross-sectional HRTEM and SAED micrographs of the interface between the

10% Sn-doped β-Ga2O3 film and the MgO substrate.

Fig. 6 XPS spectra of the 10% Sn-doped β-Ga2O3 film.

Fig. 7 (a) The optical transmittance spectra of β-Ga2O3:Sn samples. The plot of (αhν)2

as a function of photon energy hν are shown in the inset. (b) Eg of the β-Ga2O3:Sn

films as a function of Sn content.

Table 1

The resistivity, carrier concentration and mobility of the samples with 10% and 11% Sn doping levels

Tin concentration

(mol%)

Resistivity

ρ(Ω·cm)

Carrier concentration

n(cm-3

)

Mobility

µ(cm2V

-1s

-1)

10% 5.21×10-2

3.71×1019

3.35

11% 4.99×10-1

4.34×1018

2.88


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Fig. 1 XRD spectra of the Ga2O3 :Sn films with different doping levels. 79x78mm (600 x 600 DPI)


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Fig. 2 The resistivity of the Ga2O3 films with different Sn concentrations. 57x41mm (600 x 600 DPI)


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Fig. 3 Resistivity (ρ), Carrier concentration (n) and Hall mobility (µ) of β-Ga2O3 films with 10% Sn doped as a function of reciprocal temperature.

56x40mm (600 x 600 DPI)


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Fig. 4 The SEM image for the 10% Sn-doped β-Ga2O3 film: (a) the cross-section area and (b) surface. 160x217mm (300 x 300 DPI)


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Fig. 5 Cross-sectional HRTEM and SAED micrographs of the interface between the 10% Sn-doped β-Ga2O3 film and the MgO substrate. 72x64mm (300 x 300 DPI)


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Fig. 6 XPS spectra of the 10% Sn-doped β-Ga2O3 film. 104x78mm (600 x 600 DPI)


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Fig. 7 (a) The optical transmittance spectra of β-Ga2O3:Sn samples. The plot of (αhν)2 as a function of photon energy hν are shown in the inset. (b) Eg of the β-Ga2O3:Sn films as a function of Sn content.

119x179mm (600 x 600 DPI)


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Electrical and optical characterizations of β-Ga 2 O 3 ...

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Transcript of Electrical and optical characterizations of β-Ga 2 O 3 ...